Array gives correct output for this mergesort function but vector is giving incorrect output. What is going wrong?

Question

I've been trying to do the count inversions question using mergesort for the past 2-3 days and after much trying, I just picked up the answer from Hackerrank's editorial, now their code is using an Array , and if I use a Vector instead of an Array , the answer is Actual answer + 1 (or different to say the least haven't tried it on many cases). I was wondering what might be the reason for it.

I also have another question on explanation of this code, in particular the variable declarations and their use in the mergei function. I understand the rest of the code conceptually, but because of this part, I have some confusion.

    int ni = ((i+j)/2) + 1, nj = j + 1;
    int s = i;
    int* arr = new int [j - i + 1];
    j = ni; int k = 0;

Code:

void mergei(int a[], int i, int j) {
    int ni = ((i+j)/2) + 1, nj = j + 1;
    int s = i;
    int* arr = new int [j - i + 1];
    j = ni; int k = 0;

    while(i < ni && j < nj) {
        if(a[i] <= a[j]) {
            arr[k++] = a[i++];
        } else {
            arr[k++] = a[j++];
            ans += (ni-i);
        }
    }

    for(; i < ni; i++, k++) arr[k] = a[i];
    for(; j < nj; j++, k++) arr[k] = a[j];
    for(k = 0; s < nj; s++, k++) a[s] = arr[k];
    delete [] arr;
}

void m_sort(int a[], int i, int j) {
    if(i < j) {
        m_sort(a, i, (i+j)/2);
        m_sort(a, ((i+j)/2) + 1, j);
        mergei(a, i, j);
    }
}

int main() {
    // vector<int> a = {2, 1, 3, 1, 2};
    int a[] = {2, 1, 3, 1, 2};
    // int n = a.size();
    int n = sizeof(a)/sizeof(a[0]);
    m_sort(a, 0, n - 1);
    cout << ans << endl;
    return 0;
}

Answer 1

我没有通过引用传递 Vector，在数组的情况下我不必担心。